The purpose of the principal objective lens in optical disc readers is to project a light spot of specified size (sometimes the minimum attainable) on an optically responsive information recording plane. This requires that the distance from objective to filmplane be accurately set for best focus and maintained there, even in the presence of physical disturbances. In the case of an optical read-only memory (OROM), these disturbances include axial runout and disc warpage, disc thickness variations, and environmentally-induced variations of critical optical distances. In order to compensate the optical system for these disturbances, an automatic focus adjustment system must generate a focus error signal and physically adjust the optical system to restore optimum focus.
In the prior art, including video disc devices, access to a particular data location is typically provided by a lead-screw or rack-and-pinion driven sled, which carries the entire optical system. The autofocus mechanism consists mainly of an electromagnetic or a pneumatic positioner and the principal objective lens. The positioner adjusts the axial distance between lens and information recording plane in response to a control input which is developed from the focus error signal. Such an arrangement is not usable on fast optical memory discs because it cannot satisfy the requirements for both rapid random data access and focus tracking at high rotational disc speeds.
In U.S. Pat. No. 4,051,529, entitled, "Focus Control System with Movable Mirror", the autofocus mechanism consists of a movable mirror located in the optical path between a light source and the principal objective lens. Movement of the mirror in response to a focus error signal compensates for variations in the critical optical pathlengths. In the prior art device, however, the total optical distance between movable mirror and optical recording medium is fixed, except for minor variations caused by the mirror motion. This is necessitated by the use of converging lenses in the system which establish an intermediate focal point between movable mirror and optical recording medium. Thus, in this prior art device, it is required that the entire optical system be carried on the sled. Such an arrangement would not be suitable for a random-access system, such as an OROM, since the mass of the optics would prevent the rapid lateral accessing motions of the sled required to achieve random access.